Opening device for a packaging container and method of manufacturing the same

ABSTRACT

Opening device for a packaging container with an unbroken, liquid-tight pouring spout (31, 33) and a closure part (3) is disclosed. A method of manufacturing the opening device entails forming the pouring spout (21, 33) with the closure part (3) as a mold, with a heated, thermoplastic material layer being forced into the closure part through a pressure difference.

The present invention concerns an opening device for a packagingcontainer, and this opening device includes a closure part and a pouringspout. The invention also concerns a method of manufacturing an openingdevice for a packaging container.

Packaging containers of disposable type are now used to a great extentfor packaging consumer goods of different types, e.g. drinks such asmilk and juice. For reasons of cost the packaging containers are usuallymade of relatively thin packaging laminates which contain astrength-giving layer of a fibrous material, e.g. paper, which islaminated on both sides with liquid-tight layers of thermoplastic, e.g.polythene. Such packaging laminates are flexible and can be cut and bentto the desired shape. The external coating of thermoplastic materialalso makes it possible to heat-seal the laminate to itself in a simplemanner, so that a packaging container of the desired shape can beobtained.

It is naturally desirable that the finished packaging container shouldinclude an opening device which makes it possible to open the packagingcontainer in a simple manner when the contents are to be consumed. Theopening device must further make it possible to extract the packagingcontainer's contents in liquid form in a well collected stream withoutthe risk of spillage or dripping. These objectives have, however, shownthemselves to be difficult to combine in a simple manner with packagingcontainers made of flexible paper and plastic laminate, since thepackaging laminate can only with difficulty be pulled up and shaped to apouring spout or other form which gives the desired stable flow ofcontents. The shaping of an opening device directly in the packaginglaminate furthermore makes it difficult to provide a liquid-tightreclosure of the packaging container. In order to avoid theabovementioned difficulties a known method is to manufacture a packagingcontainer whose body mainly consists of the abovementioned packaginglaminate of paper and plastic, but whose top part is manufactured from acompletely plastic material through injection moulding of an upper endgable in direct contact with the laminate. The end gable can by thismeans be provided with an opening device of the desired form, which inaddition can have some form of reclosure possibility. This type ofpackaging container is shown e.g. in the German patent DE 32 17 156.Packaging containers of this kind have proved very useful from theconsumer's point of view and are simple to open and reclose. Theinjection moulding of the packaging container's upper end gable,however, entails that the packaging machine must be provided withfurther costly equipment at the same time as the machine's capacity islimited by the fact that the injection moulded upper part, owing to itsrelatively great thickness of material requires a certain cooling andstabilisation time before the packaging container can be transportedfurther through the packaging machine to the subsequent work stations.

An aim of the present invention is to ensure an opening device for apackaging container, with this opening device being simple tomanufacture and containing a closure part and a pouring spout, which canbe used with packaging containers of known type and thereby avoid theabove disadvantages.

A further aim of the present invention is to ensure an opening device ofthe abovementioned type which gives the packaging container good pouringproperties and at the same time ensures a safety closure for thepackaging container.

A further aim of the present invention is to ensure an opening devicewith a properly functioning closure part, which is easy to open andreclose in accordance with established principles well known to theconsumer. A further aim of the present invention is to ensure an openingdevice for packaging containers, with this opening device using aminimum of material and thereby resulting in a very low cost.

The above and other aims have been achieved according to the inventionthrough the fact that a packaging container of the type named in theintroduction is given the characteristics that the closure part of theopening device engages form-lockingly with and encloses the end of thepouring spout facing away from the packaging container, with this endconsisting of a connected, liquid-tight layer of material which isshaped according to the internal shape of the closure part and arrangedso as only to be pierced in connection with the opening of the openingdevice.

Existing embodiments of the opening device according to the inventionhave further been given the characteristics which can be seen from thesub-claims 2-9.

An aim of the present invention is further to ensure a method ofmanufacturing an opening device for a packaging container, with thismethod not being affected by the abovementioned disadvantages but makingit possible quickly and at low cost to apply an opening device suitablefor the purpose both with known and new types of packaging container.

A further aim of the present invention is to ensure a method ofmanufacturing an opening device, with this method at one moment forminga pouring spout and providing the pouring spout with a closure part.

A further aim of the present invention is to ensure a method which makesit possible to provide a packaging container with an opening devicewithout introducing as a result manufacturing or cooling factors whichreduce the rate of manufacture.

The above and other aims have been achieved according to the inventionthrough the fact that a method of the type named in the introduction hasbeen given the characteristics that a thermoplastic material layer isexposed to a pressure difference, which brings it to bear against in aformable state and to engage form-lockingly with a prefabricated part ofthe opening device contained in a forming tool.

Existing embodiments of the method according to the invention havefurther been given the characteristics which are clear from sub-claims11-17.

A proposed embodiment of both the method and the device according to theinvention will now be described in greater detail with reference to theenclosed schematic drawings which only show the details indispensablefor understanding the invention.

FIG. 1 shows in perspective an opening device according to the inventionon a packaging container of known type.

FIG. 2 shows an opening device according to the invention on anothertype of packaging container.

FIG. 3 A-E shows step by step and partially in cross section theapplication of an opening device according to the invention on apackaging container as per FIG. 1.

FIG. 4 A-D show step by step and in cross section the shaping of apackaging container according to the invention on a packaging containerof the type which is shown in FIG. 2.

The opening device according to the invention and the method ofproducing the same can be used with a number of types of packagingcontainer of disposable character, e.g. packagings of the type which ismanufactured from a laminated material containing layers of paper andthermoplastic. Such packaging containers are generally used for contentsin liquid form, e.g. milk or juice, and have hitherto most frequentlybeen provided with an opening device which comprises a break-up ortear-off part of the packaging container, which after opening is given amore or less pronounced pouring spout shape. In order to facilitate theopening, improve the keeping quality and make possible a liquid-tightreclosure, according to the invention an opening device is provided anda method of producing the same which can replace the previously named,known arrangement and methods in conventional packaging containers butcan on the other hand also be used to create new types of packagingcontainers.

FIG. 1 shows a known packaging container 1 of what is known as gable-toptype, which is manufactured from a laminated material which contains abearer layer of paper, which is coated on both sides with thermoplasticmaterial, e.g. polythene. The packaging material has been given thedesired form and been folded and heat-sealed for formation of apackaging container which is mainly rectangular in cross section with anupper part of what is known as gable-top type. This known packagingcontainer 1 has been provided with an opening device 2 according to theinvention, with this opening device being placed in one of the twosloping gable panels 7 of the upper part. The opening device has aclosure part 3, which is shaped like a screw stopper of mainlyconventional type.

The opening device and the method of manufacturing the same can also beused with other types of packaging container, e.g. the packagingcontainer 4 shown in FIG. 2, which has a mainly square cross sectionwith rounded corners. The packaging container's bottom surface can be ofconventional type and e.g. consist of a number of bottom surfaces foldedand sealed to each other in a liquid-tight manner, of the type that isgenerally used also with gable-top packagings. The upper part of thepackaging container 4 contains the opening device 2 according to theinvention, and this opening device has a pouring spout which is notvisible and which is formed of and constitutes an integrated part of theupper end wall 5 of the packaging container 4. The upper end wall 5 ismanufactured of a material which consists wholly of or contains layersof thermoplastic, preferably polythene. The upper end wall 5 is bentdown over and sealed to the jacket surface 6 of the packaging container4, which like the packaging container in general can consist of apackaging laminate of the abovementioned type or naturally bemanufactured from some other kind of material. Apart from the upperparts with the opening device 2 the packaging containers do not form anypart of the invention but can be of conventional type, and thereforethey are not described in greater detail in this connection.

The detailed forming of the packaging containers and especially of theopening devices can be better seen from FIGS. 3 and 4, where therepresentation of two different preferred embodiments of the openingdevice according to the invention is described in greater detail.

FIGS. 3 A to 3 E incl. show schematically the various stages in themanufacture of an opening device for a packaging container of the typewhich is illustrated in FIG. 1, i.e. what is known as a gable-toppackaging. The opening device 2 according to the invention is applied onthe packaging container before it has been closed, since it is necessaryfor both the inside and the outside of the packaging material to beaccessible during the shaping of the opening device. As can be seen fromFIGS. 3 A to 3 E incl. the packaging container is complete, but not yetfilled with contents or closed at the top, and therefore the wall panelswhich form the upper part of it extend vertically upwards and form acontinuation of the side walls of the packaging container 1, which givesmaximum accessibility so that tools for shaping and applying the openingdevice according to the invention can be introduced into the upper partof the packaging container.

The packaging container in FIG. 3 A has a mainly conventional shape, butin one of its two opposing top panels 7 a circular part of the packagingcontainer material has been removed and replaced with a disc 8 ofthermoplastic material, e.g. polythene. The disc 8 bears against theinside of the packaging material and is given a liquid-tight seal to thethermoplastic inside layer of the packaging material through a heat seal(not visible in the figure) extending in annular form around theopening. In FIG. 3 B it is shown how the thermoplastic material disc 8is exposed to heating to softening temperature (with polythene c. 110°C.) with the aid of a heating device 9, which includes two legs 10, 11which carry heater bodies 12, 13 at their outer ends which face eachother. The heater bodies 12, 13 are mainly circular and bowl-shaped witha diameter which slightly exceeds the diameter of the disc 8. The heaterbodies contain heating elements, e.g. electrical resistance material orchannels for hot air (not shown in the figure) for heating the plasticdisc in a well known manner over the whole of its area corresponding tothe hole in the packaging container wall. As can be seen from FIG. 3 Bone leg 10 is brought down through the upper open end of the packagingcontainer, while the other leg 11 is placed parallel to the first namedleg but on the outside of the packaging container wall. The legs 10, 11can be brought towards each other like a pair of tongs so as to placethe heater bodies 12, 13 at a suitable distance from the material disc 8during the heating process.

When the material disc 8 is heated to the desired temperature with theaid of the heating device 9 the next period begins, which is illustratedin FIG. 3 C. In this a forming tool 14 is used, which with two legs 15,16 like the heating device 9 can be brought down like a pair of tongsinto the open end of the packaging container and outside the wall of thepackaging container respectively and grip the thermoplastic material 8.One leg 15 contains a mainly circular, bowl-shaped cut-out 17 which isconnectable via a channel 18 with a compressed air source (not shown).The opposite leg 16 is provided on its working surface facing the leg 15with a cut-out 19, which is connectable via a channel 20 with theambient air. The cut-out 19 is mainly circular and so dimensioned that aclosure part 3 (not visible in FIG. 3 C) introduced into the cut-out 19remains in the cut-out during the use of the forming tool 14. When theforming tool 14 is placed in the position shown in FIG. 3 C the legs 15,16 are brought together so that they enclose the packaging containerwall 7 and the material disc 8 located in it which is heated tosoftening temperature. As soon as the part of the leg 15 provided withthe cut-out 17 is brought to bear sealingly against the inside of thepackaging container wall 7 and the leg 16 is brought into contact withthe outside of the wall 7 in a corresponding manner the cut-out 17 isconnected via the channel 18 with the compressed air source so that anoverpressure occurs, which acts on the soft thermoplastic disc 8 so thatit is pressed into and shaped according to the inside of theprefabricated closure part 3. The air which is in the closure part 3streams out via the cut-out 19 and the channel 20 into the ambient air,which makes it possible to bring the thermoplastic disc 8 to accurate,form-locking connection with the threaded or beaded inside of theclosure part 3. At the same time as the material in the thermoplasticdisc 8 cools down, the legs 15, 16 are moved away from each other andthe forming tool 14 is lifted out of engagement with the packagingcontainer. The closure part 3 consequently slides out of the cut-out 19and remains, thanks to the form-locking engagement with the now shapedmaterial disc 8, on the outside of the packaging container. In FIG. 3 Dthe packaging container is shown after the forming tool 14 has beenremoved, and from the figure it can be seen how the central part of thematerial disc 8 now forms a pouring spout 21 situated inside the closurepart 3, with this spout being provided with threads which have beenformed in accordance with the threaded internal surface of the closurepart 3. From FIG. 3 D it can also be seen how the material in thematerial disc 8 forms a connected, liquid-tight layer of material fromthe liquid-tight, annular connection with the inside of the packagingcontainer wall 7 to the outer, closed end of the pouring spout 21, whichis arranged only to break in connection with the opening of thepackaging container. In FIG. 3 E the finished packaging container isshown, which, after being filled with the contents, is provided, bymeans of folding and heat sealing, with what is known as a gable-shapedupper part, in which the opening device 2 according to the invention issituated mainly centrally on one top surface. The packaging container isshown in the open state, i.e. the closure part 3, which has the form ofa screw stopper, has been unscrewed from the threaded pouring spout 21.Through heat sealing the outer end of the pouring spout 21 to the insideof the closure part 3 an automatic penetration of the outer end of thepouring spout 21 can be effected so that the contents of the packagingcontainer automatically become accessible for pouring out. This will bedescribed in greater detail below. FIGS. 4 A to 4 D incl. show the workphases in the manufacture of an opening device 2 according to theinvention with an upper end wall 5 on a packaging container of the typethat is shown in FIG. 2. This packaging container's mainly tubularjacket wall 6 is at least externally coated with a thermoplasticmaterial. From FIG. 4 A it can be seen how the jacket wall 6 or at leastthe upper end of it is placed in an annular holder 22, whose centralhole has a shape and size corresponding to the jacket wall. The holder22 has an upper, flat work surface 23 against which a material disc 24manufactured from a thermoplastic, e.g. polythene, is placed. Thematerial disc 24 is held fast against the work surface 23 of the holder22 with the aid of a forming tool 25, whose lower forming surface 26bears against the top side of the material disc 24. The forming surface26 is provided with a cut-out 27, in which a prefabricated closure part3 is placed. The closure part 3 comprises a gable wall 28 and a jacketwall 29 and it can be shaped like a screw stopper with internal threadsor a snap-on stopper with internal beading (not shown). The jacket wallof the closure part 3 exhibits a channel in the form of a hole 30, whichvia suitable channels 31 in the forming tool 25 connects the inside ofthe closure part 3 with the ambient air. The closure part placed in theforming tool 25 is so turned that its open end is directed downwards andthe lower edge of the jacket wall 29 thus bears against the uppersurface of the material disc 24. The material disc 24 which has beenpreviously heated to softening temperature is now subjected to a shapingtreatment, which is preferably effected through a pressure differencebeing created between the two sides of the material disc 24. The inside4 of the packaging container (i.e. the not yet closed bottom end of thepackaging container) is suitably connected with a compressed air sourcewhich is not shown, at the same time as the forming surface 26 andcut-out 27 of the forming tool 25 are connected via the channels 31 withthe ambient air. The pressure difference obtained by this means acts onthe material disc 24 from underneath so that it is pressed against theforming surface 26 and the prefabricated closure part 3 which is in thecut-out 27 and which is maintained at room temperature or at any rate atsuch a temperature that no shaping effect of the material takes place.Since the air which is inside the closure part 3 can freely flow outthrough the channels 31 in the forming tool 25 it does not offer anyresistance to the central part of the material disc 24, but the lattercan press in and be shaped according to the inside surface of theclosure part so that it engages form-lockingly with threads 32 orbeadings, which are not shown, in the closure part 3. Surrounding partsof the material disc 24 are pressed against the forming surface 26,which can be flat or profiled to the desired pattern to give the upperend wall 5 of the packaging container the corresponding shape. As soonas the shaping has been completed the supply of compressed air to theunderside of the material disc 24 is cut off and the forming tool 25removed. At this the closure part 3 remains on the pouring spout 33formed by the central part of the material disc 24 and provided withthreads 32. The holder 22 can now also be removed and an annular part ofthe material disc 24 extending beyond the jacket wall 6 can be foldeddown and heat sealed to the outside of the jacket wall. This can ifdesired also be done at an earlier stage, e.g. in connection with theshaping of the pouring spout. The now shaped pouring spout 23 located inthe closure part 3 consists of an integrated part of the connected,liquid-tight layer of material which also forms the upper end wall 5. Inorder to make possible an upwards breaking of the upper end of thepouring spout 33 in conjunction with the removal of the closure part 3an annular seal is provided between the upper end surface of the pouringspout 33 and the inside surface of the gable wall 28 of the closure part3. This is done with the aid of a sealing tool 34, which is mainlycircular and has at its upper end an annular work surface which can beheated to sealing temperature. Through pressing the sealing tool 34against the inner end surface of the pouring spout an annular sealingtogether of the material in the pouring spout and the closure partoccurs, so that the closure part can only be removed from the upper partof the packaging container through simultaneous upwards breaking of theend surface of the pouring spout 33 along the annular sealing line. Withthe opening of the packaging container according to the invention thescrew stopper or closure part 3 is unscrewed, whereupon the end of thepouring spout sealed to the inner surface of the closure part comes awayfrom the pouring spout and follows the screw stopper, which isillustrated in FIG. 4 D. Since the liquid-tight material in thethermoplastic disc 24 is thus broken only in conjunction with theremoval of the closure part 3 it is ensured that the opening deviceremains completely leakproof, at the same time as the construction givesa guarantee against inadvertent or unauthorised opening.

When a packaging container of known type, e.g. the gable-top type shownin FIGS. 1 and 3, is to be provided with an opening device according tothe invention the thermoplastic material disc 8 can be applied throughheat-sealing on the packaging material already before the latter isshaped into a packaging container, i.e. while it is still in strip orsheet form. The shaping of the packaging material into individualpackaging containers can be done after that in the usual manner and withconventional packaging machines right up to the time that the packagingcontainer open at the top is to be filled with the desired contents.Before the filling station one or more stations are inserted forcarrying out the procedure according to FIGS. 3 A to 3 D incl., afterwhich the filling can be undertaken in the well known manner and the toppart folded together and sealed together in a liquid-tight manner bymeans of known technology.

The type of packaging container shown in FIGS. 2 and 4 is manufacturedthrough the packaging laminate first being given a tubular shape, afterwhich the longitudinal edges of the material are sealed to each other ina liquid-tight manner so that the jacket wall 6 is formed. After thisthe jacket wall is placed in the annular holder 22 and the thermoplasticdisc 24 is placed to bear against the upper end of the jacket wall 6.The forming tool 25 with the closure part in place in the cut-out 27 isbrought into its working position, after which the forming processaccording to FIGS. 4 A and 4 B takes place. When the forming tool 25 andthe holder 22 have been removed and the operation according to FIG. 4 Chas eventually taken place the packaging container is turned upsidedown, after which it is filled via the bottom with the desired contents.Finally the bottom of the packaging container is sealed, e.g. throughfolding in and heat-sealing of the prepared bottom wall surfaces or inany other suitable manner, e.g. with the aid of a further material disc,which is heat sealed along the lower edge of the jacket wall 6.

Both the opening device and the method of manufacturing the same can ofcourse be used also with completely different types of packagingcontainer, e.g. those which are manufactured from different materialcombinations and from one or a multitude of different parts. The part ofthe packaging container which is provided with a pouring spout accordingto the invention must be manufactured from a heat-formable layer ofmaterial, but this can either constitute a limited part of the packagingcontainer or be an integrated part of the total packaging container,which in that case can be entirely manufactured through heat-forming.

When packing containers are produced from a material web, said web canwholly or partly be composed of a thermoformable material. After thereshaping of the material web into packaging containers, which can beboth filled with filling goods and sealed, a chosen part of the packingcontainer is heated in order to be shaped, by means of an externalvacuum, into a pouring spout in accordance with what is earlierdescribed. If a packaging container is sealed and thus completelyair-tight this operation will make use of a part of the head-space areain the packing container and create a certain underpressure so that theflexible walls of the container will bulge slightly inwards. This ishowever not a drawback. When a packing container of this kind is opened,which can take part in connection with the removal of the closure partor in a later, separate manoeuvre, air will be sucked into the containerso that the surface of the filling goods is lowered and the containerwalls resume their original shape. If the closure part also ismanufactured from a thermoplastic material it is conceivable to formthis at the same time as the pouring spout, but with this an outerforming part is necessary in order to give the closure part a correctexternal shape.

The internal shape of the closure device must accord with its externalshape (the material must be thin), and the internal shape thereby givesthe pouring spout its corresponding shape. A precondition for the abovedescribed procedure is of course that the plastic material in theclosure part is prevented from sealing to the pouring spout material, atany rate over the greater part of its surface. This can be achievedthrough suitable selection of material, or through coatings withseal-inhibiting layers.

As previously mentioned, some form of outlet channel 30 is required frominside the closure part 3 in order to prevent air which is in theclosure part inhibiting the formation of the pouring spout 21, 33. It isnaturally undesirable that the channel should have such a positioningthat it causes leakage on reclosure of an opened packaging containerwith the aid of the closure part 3, and the end of the channel comingout inside the closure part can therefore not be placed in the gablewall 28 of the closure part, which at first sight appears necessary inorder to guarantee the escape of the air enclosed during the shapingprocess. Practical tests have, however, shown that a perfectlysatisfactory shaping is ensured even if the end of the channel comes outin the jacket wall 29, since the material in the disc 8, 24 during theshaping process presses the air down from the centre at high speed andto the outside. An optimum compromise between good shaping and minimumleakage has been shown to be obtained if the end of the channel comingout inside the closure part is situated mainly half-way up the jacketwall, i.e. at the same distance from the gable wall 28 and the loweredge of the jacket 29 facing the packaging container.

The channel does not need to be formed as a hole 30 penetrating thejacket wall 29, but can also consist of a slot extending vertically inthe jacket's inner wall surface, which via a corresponding slot runningradially in the edge surface of the jacket has connection with theambient air.

I claim:
 1. Method of manufacturing an opening device for a packagingcontainer, comprising the steps of:aligning an open end of aprefabricated part of the opening device with a thermoplastic layer ofmaterial; and exposing the thermoplastic material to a pressuredifference such that the thermoplastic material is shaped according toan interior area of the prefabricated part of the opening device. 2.Method according to claim 1, comprising the further step of heating thethermoplastic material to it softening temperature before exposing thethermoplastic material to the pressure difference.
 3. Method accordingto claim 2, comprising the further step of maintaining the prefabricatedpart at a temperature below the softening temperature of thethermoplastic material.
 4. Method according to claim 1, comprising thefurther steps of surrounding the prefabricated part and thethermoplastic material with a forming tool, and developing the pressuredifference by evacuating air.
 5. Method according to claim 4, whereinthe prefabricated part is formed with a outlet channel and air isevacuated from the interior area of the prefabricated part through theoutlet channel.
 6. Method according to claim 1, comprising the furthersteps of:surrounding the prefabricated part of the opening device andthe thermoplastic material with a forming tool; and removing theprefabricated part of the opening device from the forming tool and, atthe same time, removing the shaped thermoplastic material from theforming tool.
 7. Method according to claim 1, comprising the furtherstep of connecting, with means including at least the pressuredifference, the thermoplastic material in a liquid-tight manner withadjacent parts of the packaging container.
 8. Method according to claim7, comprising the further step of covering, with the thermoplasticmaterial, an opening in the packaging container, the area of the openingin the packaging container being mainly of the same size as the open endin the prefabricated part.
 9. Method according to claim 8, comprisingthe further step of enclosing edges of the packaging container in aliquid-tight manner with the thermoplastic material.
 10. Methodaccording to claim 7, comprising the further step of enclosing edges ofthe packaging container in a liquid-tight manner with the thermoplasticmaterial.
 11. Method according to claim 1, comprising the further stepof joining the thermoplastic material in a liquid-tight manner to apackaging container prior to exposing the thermoplastic material to thepressure difference.
 12. Method according to claim 11, comprising thefurther step of covering, with the thermoplastic material, an opening inthe packaging container, the area of the opening in the packagingcontainer being mainly of the same size at the open end in theprefabricated part.
 13. Method according to claim 11, comprising thefurther step of enclosing edges of the packaging container in aliquid-tight manner with the thermoplastic material.
 14. Methodaccording to claim 1, wherein the prefabricated part is formed with anoutlet channel and the pressure difference is developed by evacuatingair from the interior area of the prefabricated part through the outletchannel.
 15. A method of manufacturing an opening device for a packagingcontainer, comprising the steps of:aligning a first surface of a disc ofthermoplastic material with an open end of a cap having an interiorshape defined by substantially cylindrical side walls and a top wall;and applying gas pressure against a second side of the disc so that thedisc forms a spout conforming to the interior shape of the cap.
 16. Themethod according to claim 15, comprising the further step of heating thedisc to a softening temperature before applying the gas pressure againstthe second side of the disc.
 17. The method according to claim 15,comprising the further steps of aligning edges of the disc with edgesaround an opening in a wall of a packaging container and forming anannular, liquid-tight seal between the edges of the disc and the edgesaround the opening.
 18. The method according to claim 17, wherein theliquid-tight seal is formed by heat sealing the edges of the disc to theedges around the opening.
 19. The method according to claim 18, whereinpressure is applied to the edges of the disc.
 20. The method accordingto claim 19, wherein the pressure is at least partially the gas pressureapplied against the second side of the disc.
 21. The method according toclaim 18, wherein the liquid-tight seal is formed before applying thegas pressure against the second side of the disc.